Which monster do you like the most in Heroes 5 at level 7?

You're quite mistaken.
The Necropolis (gray) dragon has decent attack and defense (30 and 28), and its total HP is also acceptable (there are 2 of them, not 3) - 480. It doesn't inflict aging, but applies a weakening effect. However, its damage is indeed small, but the worst thing is that it's slow (speed 7, not 9 like others) and doesn't even reach large creatures. If it were faster, considering its initiative of 11, it could be used, but as it is, in my opinion, it's an absolute loser.